The transportation of chemicals in the liquid or gas state in pipes presents numerous advantages: it is more economic, once the infrastructure has been put in place; it makes possible the transportation of large volumes; it provides great safety of supply, by virtue of a uniform flow rate; finally, it is a safer means than rail or road.
It is known to use, for the transportation of liquid or gas products, metal pipes or pipes made of plastics, or also metal pipes coated with one or more polymer layers. According to the fluid to be transported, these pipes have to meet multiple requirements, in particular as regards the properties of mechanical strength (in particular impact strength), of elasticity, of creep strength, of fatigue strength, of resistance to swelling, of chemical resistance (resistance to corrosion, to oxidation, to ozone, to chlorinated products, and the like) and of thermal resistance.
For example, pipes are known which comprise one or more metal components which guarantee the mechanical stiffness but which are not leaktight to the fluids transported (for example components made of steel or of cast iron), and also various layers based on polymeric compositions, in order to provide leaktightness to the fluids transported and also thermal insulation. Typically, in the case of the polymeric layers, the thickness/diameter ratio is of the order of 1/10. These polymeric compositions can be polyethylene-based but this limits the temperature of use of the pipes to a maximum of 60° C. They can also be based on fluoropolymers, such as polyvinylidene fluoride (PVDF), suitable for higher temperatures of use, up to 130° C., and exhibiting good chemical resistance and good thermal behavior. However, PVDF is very rigid and, for this reason, PVDF homopolymers are often formulated or used as a blend with copolymers based on vinylidene fluoride (VDF) and optionally plasticizer, in order to reduce the stiffness thereof.
The document BE 832851 describes fluoroelastomers comprising a molar proportion of 50 to 85% of VDF and of 15 to 25% of hexafluoropropylene (HFP), i.e. a proportion by weight of 47 to 71% of VDF and of 29 to 53% of HFP, which are used for the manufacture of molded PVDF bodies comprising from 1 to 30% by weight of fluoroelastomer. However, such compositions have a limited extrudability and do not make possible the manufacture of pipes having a thickness/diameter ratio of approximately 1/10. Furthermore, such compositions exhibit an insufficient fatigue strength for the applications described above.
The document EP 1 342 752 describes PVDF-based compositions comprising: (A) a PVDF homopolymer or a VDF-based copolymer; (B) a fluoroelastomer; (C) optionally a plasticizer. The fluoroelastomer (B) is present at a content of from 0.5 to 10 parts by weight per 100 parts of homopolymer or copolymer (A) and from 0 to 10 parts by weight of a plasticizer (C), with the additional condition that the sum of (B) plus (C) is from 0.5 to 10.5 parts by weight. These compositions correspond to the following proportions by weight: 89.5 to 90.5% of a PVDF homopolymer or a VDF-based copolymer (A); 0.5 to 9% of a fluoroelastomer (B); 0 to 9% of a plasticizer (C). Disclosed among the examples are compositions comprising from 2 to 4% of VDF/HFP copolymer as fluoroelastomer. The HFP content in the copolymer is from 30 to 40%.
The document EP 0 608 639 describes polymeric compositions comprising, by weight, from 60 to 80% of PVDF, from 20 to 40% of a thermoplastic copolymer of VDF and of another fluorinated comonomer (present at a content of 5 to 25% in the copolymer) and from 5 to 20% of a plasticizer (with respect to the sum of the PVDF and copolymer). The thermoplastic copolymers envisaged include VDF/HFP copolymers. The HFP contents indicated in the copolymers which are disclosed in the examples are of the order of 10%.
The document EP 0 608 940 describes polymeric compositions comprising, by weight, from 25 to 75% of PVDF and from 25 to 75% of thermoplastic copolymer of VDF and of another fluorinated comonomer (present at a content of 5 to 25% in the copolymer). The thermoplastic copolymers envisaged include VDF/HFP copolymers.
The document WO 2006/045753 describes polymeric compositions based on PVDF and on a thermoplastic fluorinated copolymer. A plasticizer can added at a content of at most 5%. The proportion of thermoplastic fluorinated copolymer is, for example, from 20 to 55%, with respect to the sum of the latter and of PVDF. The thermoplastic fluorinated copolymer can, for example, be a copolymer of VDF and of another fluorinated comonomer, which can be present in a content of 5 to 25%. HFP is mentioned as possible fluorinated comonomer.
The document WO 2007/006645 describes compositions comprising a thermoplastic fluoropolymer (which can, for example, be a blend of VDF homopolymer and of copolymer of VDF and of a fluorinated comonomer present at a content of 0.1 to 15 mol % in the copolymer), a (per)fluoropolyether and a per(halo)fluoropolymer.
However, the polymeric compositions provided in the state of the art are not completely satisfactory. In particular, the fatigue strength of some polymeric compositions of the state of the art is regarded as insufficient for the applications targeted and very particularly for the manufacture of pipes for the transportation of liquid or gas synthetic products (for example for the transportation of hydrogen).
There thus exists a real need to develop an alternative polymeric composition exhibiting improved properties, in particular an improved fatigue strength, in order to manufacture pipes which remain mechanically reliable over the long term.